Using Calibre? I don't know if I'd bother trying to figure that one out.

From the K3 manual:

Transferring Audiobooks
To transfer audiobooks to your Kindle, follow these steps:
1. Connect your Kindle to your computer, using the USB cable that came with the device.
2. Your Kindle should appear on your computer in the same location you would normally find an external USB drive.
3. Open your Kindle. You should see a folder entitled “audible.” Drag any audiobooks you want to play on your Kindle into this folder.
4. Using your computer, eject your Kindle from your computer. The content you transferred should now appear in Home.